More than 20 people are confirmed dead in attacks in Kenya's coastal Lamu region, officials said Sunday, with police blaming regional separatists despite Somalia's Islamist Shebab rebels claiming responsibility.

Kenyan deputy police chief Grace Kaindi said 21 people had died in the two attacks late Saturday and blamed the Mombasa Republican Council (MRC), a group that campaigns for independence of the coastal region. 

Kenya's main opposition party is gearing up for a mass rally on Monday in a major challenge to the country's year-old government amid mounting security woes and fears of renewed ethnic violence.

Opposition leader and former prime minister Raila Odinga plans an anti-government rally on July 7 in central Nairobi, the anniversary of protests for multi-party democracy in the 1990s, a date heavy with symbolism and known commonly as "Saba-Saba" , or "Seven-Seven" in Swahili.

At least four people died when a cargo plane crashed into a building shortly after takeoff Wednesday from the Kenyan capital's main airport, the busiest in east Africa, aviation authorities said.

"A Fokker 50 cargo plane with four people on board has this morning crashed at a commercial building," after taking off from Nairobi's Jomo Kenyatta International Airport (JKIA), Kenya Airports authority said in a statement.

Al-Qaida-linked Shebab militants have the "capability and intent" to spread their attacks beyond their strongholds in Somalia, a U.N. special representative warned on Saturday, calling for greater regional cooperation to combat the threat.

The Shebab "is an organization that has a regional agenda," said Nick Kay, the U.N.'s special representative in Somalia.

The United Nations is determined to help Kenya and other countries in Africa fight "terrorism" following a series of deadly attacks, Secretary General Ban Ki-moon said Saturday.

Speaking in the capital Nairobi, he said he had talked with Kenyan President Uhuru Kenyatta "at length" about how the U.N. and Kenyan government can work together in countering "heinous" attacks.

Kenyan police have arrested the governor of the coastal Lamu district in connection with three recent massacres in which over 60 people were killed, officers said Thursday.

Governor Issa Timamy was arrested late Wednesday in connection with killings last week over two consecutive nights in the town of Mpeketoni and a nearby village that claimed nearly 60 lives. Another attack this week left at least five dead, officials said.

At least 11 people have been killed in a new attack on Kenya's coastal region, officials said Tuesday, one week after some 60 people died in twin massacres nearby.

Some were hacked to death with knives, others shot and executed at close quarters, mirroring tactics used in earlier attacks claimed by Somalia's Al-Qaida-linked Shebab.

At least 20 people have been killed in Kenya in ethnic clashes in the restive northeast, the latest in a series of revenge attacks between rival Somali clans.

Fighting broke out on Sunday between clans in the Tarbaj district of Wajir along the border with war-torn Somalia, government officials said on Monday.

Six years after Kenya erupted into ferocious ethnic battles and post-election violence, security guard John Mboya is fearful once again, after twin massacres on the coast brought political rivalries to the surface.

"When the leaders argue, it is people like me who will suffer if a fight starts," bemoaned Mboya, recalling the intensity of 2007-8 violence, when communities in his crowded slum in the capital Nairobi divided along tribal lines and turned on each other after disputed elections.

The U.S. ambassador to Kenya on Saturday moved to dismiss what he said were "false and misleading" rumors, asserting that embassy staff were not evacuating and the facility would remain open.

"A number of rumors have circulated regarding the U.S. embassy and our relationship with Kenya that are false and misleading. The U.S. embassy in Nairobi has not closed and is not closing. embassy personnel are not evacuating," Ambassador Robert F. Godec said in a statement.